Levothyroxine, also known as L-thyroxine, synthetic T4, or 3,5,3′,5′-tetraiodo-L-thyronine, CAS number 51-48-9, is a synthetic form of thyroxine, used as a hormone substitute for patients with thyroid conditions, such as hypothyroidism, as well as conditions in which the thyroid gland becomes enlarged, causing swelling of the neck.
Thyroid hormones regulate multiple metabolic processes and play an essential role in normal growth and development, and normal maturation of the central nervous system and bone. Levothyroxine sodium was initially manufactured as synthetic T4 in 1958 and it was first introduced into the market as early as before 1962 without an approved NDA, apparently in the belief that it was not a new drug.
Levothyroxine sodium is very slightly soluble in water and slightly soluble in ethanol (96 percent). Levothyroxine sodium is described in the European Pharmacopoeia. The chemical designation of Levothyroxine sodium is Sodium (2S)-2-amino-3-[4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodophenyl]propanoate. Its molecular formula is C15H10I4NnaO4, ×H2O and its molecular weight is 799 (anhydrous substance). The structural formula is:

Orally administered levothyroxine sodium is used as replacement therapy in conditions characterized by diminished or absent thyroid function such as cretinism, myxedema, non-toxic goiter, or hypothyroidism (Food and Drug Administration 1997; Wertheimer and Santella 2005).
Levothyroxine Sodium Oral Solution is indicated for:                hypothyroidism (congenital or acquired)        diffuse non toxic goitre or Hashimoto's thyroiditis        thyroid carcinoma        
The treatment of any thyroid disorder should be determined on an individual basis, taking account of clinical response, biochemical tests and regular monitoring. A pre-therapy ECG is valuable as changes induced by hypothyroidism may be confused with evidence of ischaemia. If too rapid an increase of metabolism is produced (causing diarrhoea, nervousness, rapid pulse, insomnia, tremors and sometimes anginal pain where there is latent myocardial ischaemia), reduce the dose or withhold for 1-2 days and start again at a lower dose.
Oral solutions of levothyroxine are particularly suitable for use in children and in the elderly who may have difficulty to swallow tablets. Unfortunately, solutions of levothyroxine are less stable compared to tablets during storage. Also, levothyroxine solutions may comprise relatively high amounts of liothyronine, which is believed to be the source of side-effects in certain patients. Aqueous levothyroxine solutions are prone to decomposition compared to the solid forms. The big advandage of the solution is the uniformity of dosage units in comparison to solid dosage forms (tablets). The tablets, usually due to the very low levothyroxine content (0.04% up to 0.5% w/w), have problems of content uniformity during the production process and many times the actual content that the patient receives with tablet therapy, is not 100% but could range from 85% up to 120% and this creates serious problems on patient treatment. In contrast, it is much easier to obtain a homogeneous solution.